Digitally controlled voltage generator

ABSTRACT

A digitally controlled voltage generator is disclosed for use in applications requiring fine resolution voltage control, such as generating a common voltage for a liquid crystal display. A constant resistance digital to analog converter (DAC) is configured to provide appropriate voltage steps by tuning bias resistors to generate desirable reference voltages for the DAC. The bias resistors are configured to be tuned after placement and routing steps in an integrated circuit design.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention relate generally to voltagegenerator circuits and, more specifically, to a common voltage generatorcircuit for liquid crystal displays.

2. Description of the Related Art

A modern liquid crystal display (LCD) screen is formed as an array ofpixels that are backlit using a uniform polarized light source(backlight). Each pixel comprises at least one region of liquid crystalmaterial sandwiched between two electrodes. A color LCD screen may usethree regions of liquid crystal material to form one color pixelcomprising red, green, and blue color components. The electrodes arefabricated from an electrically conductive material that is thin enoughto be relatively transparent to light, allowing light to pass throughboth electrodes and each region of sandwiched liquid crystal material.One of the two electrodes is connected to a common voltage (VCOM), andthe second electrode is connected to a column wire via a field effecttransistor that is configured to connect the column wire to the secondelectrode in response to a row select signal on a corresponding rowwire. The column wire is driven with a voltage value corresponding to adesired intensity for the associated liquid crystal region. A color LCDpanel may need three column wires, corresponding to red, green, and bluecolor components, to determine a color value for one pixel. When a givenrow is selected, a voltage potential is established between the twoelectrodes, causing liquid crystal material in a corresponding region tomodulate the polarization of light transmitted through the region. Thetransmitted light originates as polarized light from the backlight,passes through the liquid crystal material, and passes through apolarizing filter before exiting a viewing surface of an LCD panel. Bymodulating the polarization of the light transmitted through the liquidcrystal region, the pixel brightness is correspondingly modulated whenviewed from the viewing surface of the LCD panel.

Persons skilled in the art understand that the optimal VCOM voltage fora given LCD panel may vary on a panel-to-panel basis, based onmanufacturing variation for the LCD panel. In other words, VCOM for eachpanel should be individually adjusted, preferably as part of amanufacturing process. One challenge in adjusting VCOM to an optimalvalue is that relatively small differences in VCOM can cause visibledegradation in image quality of a particular LCD panel. For example, adifference of one millivolt can oftentimes have a perceptible effect onimage quality. One common type of visible degradation appears as flickerin images displayed by the LCD panel. An analog variable resistor issometimes used to adjust VCOM with millivolt resolution. However, theanalog variable resistor introduces additional manufacturing costs andis therefore not a preferred solution in high-volume LCD manufacturing.A more efficient solution involves digitally adjusting VCOM, asdescribed below in FIG. 1.

FIG. 1 illustrates a prior art digitally controlled common voltagegenerator circuit 100. The common voltage (VCOM) generator circuit 100includes a voltage generator 130, and an output driver 132, whichgenerates VCOM 190. VCOM 190 corresponds to a common voltage (VCOM)reference conventionally used in LCD panels. The voltage generator 130receives an analog voltage on node VDDA 104, and a digital voltage onnode VDDB 106. Each voltage is measured with respect to a ground (GND)node 102. The GND node 102 is defined as having a potential of zerovolts. A reference voltage associated with VREFA 112, also measured withrespect to GND 102, is generated from a resistor divider formed byresistors R 124 and R 122. VREFA 112 is generated according to a targetspecification for an associated LCD panel. Amplifier 120 is configuredto form a voltage follower with a high-impedance input, which isconnected to VREFA 112, and a low impedance output, which is connectedto VREFB 114. The voltage follower drives VREFB 114 with a low impedanceat a voltage corresponding closely to VREFA 112, thereby isolating VREFA112 from variable sink currents drawn through an “R-2R” digital toanalog converter (DAC) 140. Persons skilled in the art will understandthat an R-2R DAC 140 presents a variable current load to node VREFB 114,and that the variable current load is a function of a digital DAC value156. The DAC value 156 conventionally represents a fixed-length integer.Each integer represented by the DAC value 156 has a correspondingvoltage value generated at VDAC 150. Each increment in the DAC value 156has a corresponding voltage step at VDAC 150.

The DAC value 156 comprises a parallel vector transmitted from a serialdigital controller 144 to a DAC decoder 142. The serial digitalcontroller 144 receives a serial clock signal SCL 152 and a serial datasignal SDA 154. A digital data vector is transmitted from an externaldevice (not shown) via SCL 152 and SDA 154 for representation within thevoltage generator 130 as DAC value 156. In one embodiment, the serialdata controller 144 adheres to conventional “I2C” signaling.

In response to a given DAC value 156, the DAC decoder 142 generatescontrol signals to activate one or more analog pass gates within theR-2R DAC 140 in order to produce a corresponding output voltage at VDAC150. VDAC 150 is transmitted to the output driver 132, where amplifier160, Q 162, and R 164 are configured to convert VDAC 150 to a current,which is sourced from node 188 and sinked through R 164 to GND 102.Resistor R 184 forms the top of a voltage divider, while R 182, Q 162and R 164 form the bottom of the voltage divider, which is configured togenerate a voltage on node 188 that is between VDDA 104 and GND 102.Amplifier 180 is configured as a voltage driver (follower), which drivesVCOM 190 with a voltage corresponding closely to the voltage on node188. Amplifier 180 should be configured to drive enough current tomaintain a relatively stable voltage value on node VCOM 190.

One problem with prior art designs for the common voltage generatorcircuit 100 is that amplifier 120 is costly in terms of die area andpower consumption. Additionally, amplifier 120 introduces an offsetvoltage between VREFA 112 and VREFB 114, that may commonly correspond todozens of voltage steps at VDAC 150, thereby degrading accuracy andcontrol in the prior art common voltage generator circuit 100.Additional die area or additional power consumption, or both, may beutilized as part of a design trade-off to attempt to reduce the offsetvoltage associated with amplifier 120. However, such trade-offs furtherreduce the efficiency of the overall common voltage generator circuit100.

As the foregoing illustrates, what is needed in the art is a techniquefor precisely generating a digitally controlled voltage that is moreefficient than in existing art.

SUMMARY OF THE INVENTION

One or more embodiments of the invention provide a digitally controlledvoltage generator for use in applications requiring fine resolutionvoltage control, such as generating a common voltage for a liquidcrystal display. A constant resistance digital to analog converter (DAC)is configured to provide appropriate voltage steps by tuning biasresistors to generate desirable reference voltages for the DAC. The biasresistors are configured to be tuned after placement and routing stepsin an integrated circuit design.

A voltage generator according to an embodiment of the invention includesa constant resistance digital-to-analog converter for generating anoutput voltage, wherein the constant resistance digital-to-analogconverter includes a resistor network and is connected to a referencevoltage through a first resistor and to ground through a secondresistor. The resistor network may be a linear string of seriallyconnected resistors or a plurality of resistor segments, each having astring of serially connected resistors.

A voltage generator according to another embodiment of the inventionincludes a constant resistance digital-to-analog converter forgenerating an output voltage, a resistor divider including a firstresistor and a second resistor connected in series between a digitalreference voltage and ground, a third resistor connected at a first endto a node between the first resistor and the second resistor and at asecond end to the constant resistance digital-to-analog converter, and afourth resistor connected in series between the constant resistancedigital-to-analog converter and ground. According to a furtherembodiment, the constant resistance digital-to-analog converter includesa plurality of gates that are controlled to generate the output voltageat a desired level.

An LCD display device according to an embodiment of the inventionincludes a plurality of LCD elements to which is supplied a commonvoltage reference, and a common voltage reference generator including aconstant resistance digital-to-analog converter having a resistornetwork for generating an output voltage from which the common voltagereference is generated. According to a further embodiment, the commonvoltage reference generator further includes an output driver forreceiving the output voltage from the constant resistancedigital-to-analog converter and generating the common voltage reference.

A method of tuning a voltage generator for a digital-to-analog converterthat includes a constant resistance digital-to-analog converter,according to an embodiment of the invention includes the steps ofadjusting a resistor value of the first resistor, and adjusting aresistor value of the second resistor, wherein the resistor values areadjusted based on parasitic resistance values determined prior to thesteps of adjusting.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 illustrates a prior art digitally controlled common voltagegenerator circuit;

FIG. 2 illustrates an improved digitally controlled common voltagegenerator circuit, according to one embodiment of the present invention;

FIG. 3A illustrates an exemplary constant resistance digital to analogconverter employing a linear architecture, according to one embodimentof the present invention;

FIG. 3B illustrates an exemplary constant resistance digital to analogconverter employing a segmented architecture, according to oneembodiment of the present invention;

FIG. 4A depicts a liquid crystal display (LCD) subsystem, configured toimplement one or more aspects of the present invention; and

FIG. 4B illustrates an LCD display device, configured to implement oneor more aspects of the present invention.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth toprovide a more thorough understanding of the present invention. However,it will be apparent to one of skill in the art that the presentinvention may be practiced without one or more of these specificdetails. In other instances, well-known features have not been describedin order to avoid obscuring the present invention.

FIG. 2 illustrates an improved digitally controlled common voltagegenerator circuit 200, according to one embodiment of the presentinvention. The common voltage generator circuit 200 of the presentinvention includes a voltage generator 230, and an output driver 232,which generates a digitally controlled common voltage on VCOM 290. VCOM290 corresponds to a common voltage (VCOM) reference conventionally usedin LCD panels. The voltage generator 230 receives an analog voltage onnode VDDA 204, and a digital voltage on node VDDB 206. Each voltage ismeasured with respect to a ground (GND) node 202. The GND node 202 isdefined as having a potential of zero volts. A reference voltage on nodeVTOP 212, measured with respect to GND 202, is a function of voltageVDDA 204, R1, R2, R3, a constant resistance (RDAC) associated withconstant R DAC 240, and R4.

The DAC value 256 conventionally represents a fixed-length integer. Eachinteger represented by the DAC value 256 has a corresponding voltagevalue generated at VDAC 250. Each increment in the DAC value 256 has acorresponding voltage step at VDAC 250. The DAC value 256 comprises aparallel vector transmitted from a serial digital controller 244 to aDAC decoder 242. The serial digital controller 244 receives a serialclock signal SCL 252 and a serial data signal SDA 254. A digital datavector is transmitted from an external device (not shown) via SCL 252and SDA 254 for representation within the voltage generator 230 as DACvalue 256. In one embodiment, the serial data controller 244 adheres toconventional “I2C” signaling.

In response to a given DAC value 256, the DAC decoder 242 generatescontrol signals to activate one or more analog pass gates within theconstant R DAC 240 in order to produce a corresponding output voltage atVDAC 250. VDAC 250 is transmitted to the output driver 232, whereamplifier 260, Q 262, and R7 are configured to convert VDAC 250 to acurrent, which is sourced from node 288 and sinked through R7 to GND102. Resistor R5 forms the top of a voltage divider, while R7, Q 262 andR6 form the bottom of the voltage divider, which is configured togenerate a voltage on node 288 that is between VDDA 204 and GND 202.Amplifier 280 is configured as a voltage driver (follower), which drivesVCOM 290 with a voltage corresponding closely to the voltage on node288. Amplifier 280 should be configured to drive enough current tomaintain a relatively stable voltage value on node VCOM 290.

A design methodology for selecting values of R1, R2, R3, R4 is describedbelow. RDAC is taken as a given here, although persons skilled in theart will recognize that design trade-off also exist when designingconstant R DAC 240, and those trade-offs lead to a final value of RDAC.In general, RDAC should be a relatively large resistance value (KΩ'sthrough MΩ's) for low power operation.

Equation 1 is used to compute values for R3 and R4. RBIAS is computedand assigned to R3 (as a nominal value for R3) for the upper voltagerange of VDAC 250, and RBIAS is computed again and assigned to R4 (as anominal value for R4) for the lower voltage range of VDAC 250. RBIAS iscomputed based on RDAC, N (number of bits in DAC value 256), and α, anoffset value measured in integral (LSB) steps of DAC value 256. When R3is computed, α represents a number of steps below VREF 212 a maximumVDAC 250 voltage should be. When R4 is computed, α represents a numberof steps above 0 a minimum VDAC 250 value should be.

$\begin{matrix}{R_{BIAS} = {\frac{\alpha}{2^{N} - \alpha}R_{DAC}}} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$

Equation 2 is used to compute RDAC for the special case of a constant RDAC 240 comprising a linear resistor string. Persons skilled in the artwill recognize that other techniques, including segmented resistorstring DAC architectures, may also be used to build the constant R DAC240. Computing RDAC for DAC structures other than a series resistorstring should be computed according to the particular topology of thegiven DAC structure. For example, a segmented resistor string DAC ischaracterized by a sum of resistance values associated with a primaryrank of resistors.R _(DAC) =ΣRS  (Equation 2)

An equivalent resistance of the resistive path from VTOP 212 through theconstant R DAC 240 to ground is given in Equation 3.R _(EQUIV) =R ₃ +R ₄ +ΣRS  (Equation 3)

Equation 4 should be used to compute a resistance value for R1. Equation4 uses REQUIV computed in Equation 3, analog voltage VDDA 204, a targetvalue for VREF (VTOP 212), and value for R2. In one embodiment R2 is setto REQUIV. The target value for VREF dictates the upper voltage limitfor VDAC 250, independent of offsets implemented using bias resistors R3and R4.

$\begin{matrix}{R_{1} = \frac{\left( R_{2}||R_{EQUIV} \right)\left( {V_{DDA} - V_{REF}} \right)}{V_{REF}}} & \left( {{Equation}\mspace{14mu} 4} \right)\end{matrix}$

Using Equations 1 through 4, values for R1, R2, R3, and R4 may becomputed and used as nominal design values in a physical design(integrated circuit mask layout) implementation of the common voltagegenerator circuit 200. The physical design implementation may begenerated and advantageously used in multiple different integratedcircuit designs, each potentially with different routing constraints. Assuch, parasitic resistance between GND 202 and true ground for theintegrated circuit hosting the common voltage generator circuit 200 maycause VDAC 250 to be offset with respect to a desired design target.Similarly, parasitic resistance between VDDA 204 and the true analogsupply voltage for the integrated circuit may cause VDAC 250 to beoffset with respect to a desired design target.

In a typical physical design setting, the common voltage generatorcircuit 200 may be subject to different parasitic resistance valuesbetween VDDA 204, GND 202, and their respective reference nodes ondifferent designs. Furthermore, the parasitic resistance values are notconventionally known until after circuitry outside the common voltagegenerator circuit 200 is placed and routed within the integratedcircuit.

To compensate for parasitic resistance values outside of the commonvoltage generator circuit 200, bias resistors R3 and R4 may be adjustedin a step performed after conventional place and route has beenperformed and relevant parasitic resistance values may be determined.Each of the bias resistors R3 and R4 is configured to be independentlytunable using a small number of layer modifications. For example, R3 andR4 may be configured to be tunable by placing a via cut (one layermodification) in one or more pre-defined locations. Alternatively, R3and R4 may be independently configured by placing a poly-silicon contact(one layer modification) in a specified location. Alternatively, R3 andR4 may be independently configured by placing a poly-silicon bridge (onelayer modification) in a specified location between two strips ofpoly-silicon. Persons skilled in the art will recognize that variousother techniques for tuning R3 and R4 may be implemented withoutdeparting the scope of the present invention. Furthermore, personsskilled in the art will recognize that bias resistors R3 and R4 shouldbe designed to accommodate a range of values to compensate for a rangeof parasitic resistance values coupled to VDDA 204 and GND 202.

FIG. 3A illustrates an exemplary constant resistance (R) digital toanalog converter (DAC) 240 employing a linear architecture, according toone embodiment of the present invention. The constant R DAC 240comprises a series string of resistors RS 352, each connected to acorresponding pass gate 354. Each pass gate 354 is also coupled to VDAC250, which serves as the output node of the constant R DAC 240. Eachpass gate 354 is controlled by a corresponding control signal 350, whichis generated by a DAC decoder 244.

In one embodiment, the DAC decoder 244 activates one control signal 350at a time to turn on one pass gate 354 at a time. The DAC decoder 244activates one control signal 350 in response to receiving acorresponding DAC value 256. For example, a DAC value 256 of “1” may bedecoded by the DAC decoder 244 to activate control signal 350(1), whichcloses pass gate 354(1) to couple node 356 to VDAC 250, therebytransmitting the voltage on node 356 to VDAC 250. Each sequential nodealong the series string of resistors RS 352 establishes a discretevoltage that is linearly distributed between VBOT (the voltage on nodeVBOT 214) and VTOP (the voltage on node VTOP 212). A substantiallyidentical voltage step is established along sequential nodes. Byactivating a given pass gate 354, a corresponding discrete voltage istransmitted to VDAC 250. In one embodiment, N is an integral power oftwo, such as 2^6 (64), 2^7 (128), 2^8 (256), and so forth. When N is anintegral power of two, VDAC 250 ranges from VBOT to VTOP*(2^N−1)/(2^N).

The values of R1, R2, R3, and R4 are selected as described previously inFIG. 2. Persons skilled in the art will recognize that any constant RDAC, regardless of architecture, may be employed as constant R DAC 240without departing the scope of the present invention.

FIG. 3B illustrates an exemplary constant resistance (R) digital toanalog converter (DAC) 240 employing a segmented architecture, accordingto one embodiment of the present invention. The constant R DAC 240comprises a plurality of resistor string segments coupled to pass gatesthat are used to couple one resistor in each one of the resistor stringsegments to either another resistor string segment or to an output node.For example, resistors RS 362 comprise a first resistor string segment,and resistors RS 372 comprise a second resistor string segment. Passgates 364 connect resistors 362 the second resistor string segment viaintermediate nodes 368. Resistors 372 are connected to intermediatenodes 368 and output node VDAC 250. Each pass gate 364, 374 iscontrolled by a corresponding control signal 360, 370, generated by aDAC decoder 244 and transmitted to the constant R DAC 240 as controlsignals 350.

In one embodiment, the DAC decoder 244 activates two control signals 360at a time to turn on two pass gates 364 at a time, coupling one resistorRS 362 in parallel with intermediate nodes 368. The DAC decoder 244activates one control signal 370 at a time to couple a selected voltagefrom a final resistor string segment to output node VDAC 250. In thisexample, the second resistor string segment is the final resistor stringsegment. Each resistor string segment corresponds to bits of decreasingsignificance in the magnitude of DAC value 256. The first resistorstrong segment corresponds to the most significant bits of DAC value256, while the final resistor string segment corresponds to the leastsignificant bits of DAC value 256. Persons skilled in the art willreadily understand that an arbitrary number of resistor string segmentsmay be coupled together in this way to form a constant R DAC ofarbitrary resolution, where each resistor string segment has oneresistor coupled in parallel with a successively lower significancesegment, with the exception of the final resistor string segment, whichis coupled to the output node VDAC 250.

The control signals 350 are generated in response to receiving acorresponding DAC value 256. For each resistor string segment, a set ofcontrol bits, corresponding to bits within the DAC value 256, isgenerated within control signals 350. For example, control signals 350comprise most significant bit control signals 360 generated from twomost significant bits of DAC value 256. Control signals 350 furthercomprise least significant bit control signals 370, generated from twoleast significant bits of DAV value 256. For example, a DAC value 256 of“0001” (one linear voltage increment going from VBOT 214 towards VTOP212) should be decoded by the DAC decoder 244 to activate controlsignals 360(0), 360(1), and 370(2), which closes pass gate 364(0),364(1), and 374(2) to couple node 376 to VDAC 250, thereby transmittingthe voltage on node 376 to VDAC 250. The voltage on node 376appropriately corresponds to one linear voltage increment, going fromVBOT 214 towards VTOP 212.

The values of R1, R2, R3, and R4 are selected as described previously inFIG. 2. Persons skilled in the art will recognize that any constant RDAC, regardless of architecture, may be employed as constant R DAC 240without departing the scope of the present invention.

FIG. 4A depicts a liquid crystal display (LCD) subsystem 400, configuredto implement one or more aspects of the present invention. The LCDsubsystem 400 includes the common voltage generator circuit 200 of FIG.2, an LCD panel 430, a column driver 420 and a row driver 410. The LCDpanel 430 includes an LCD element 440(0,0), which includes a transistor,Q434, and an LCD cell 432. The LCD cell 432 includes a liquid crystalregion sandwiched between two electrodes. A first electrode of the twoelectrodes is connected to VCOM 290. A second electrode of the twoelectrodes is connected to Q 434, which is coupled to column drivesignal 424. When Q 434 is turned on, the column drive signal 424 iscoupled to the second of the two electrodes. A voltage potential betweenthe two electrodes modulates polarization rotation within the LCD cell432, which, in turn, controls how much light is transmitted through theLCD cell 432.

The row driver 410 receives a row input signal 412, and generates a rowdrive signal 414. The row input signal 412 should be a digital signalhaving two defined states, corresponding to an active state and aninactive state. In the active state, the row driver 410 drives a rowdrive signal 414 that turns on Q 434, coupling the column drive signal424 to the LCD cell 432. The column driver 420 receives an analog columninput signal 422 and drives the column input signal 422 as a voltage oncolumn drive signal 424. In one embodiment, the column input signal 422is an analog voltage signal that represents an amount of light to betransmitted by LCD element 440(0,0) when the row drive signal 414 is inthe active state. The amount of light is determined by the voltagepotential between the column drive signal 424 and VCOM 290. In oneembodiment, a software control signal 405 is used to drive signals SCL252 and SDA 254, of FIG. 2, to configure an output voltage value forVCOM 290.

FIG. 4B illustrates an LCD display device 490, configured to implementone or more aspects of the present invention. The LCD display device 490includes the common voltage generator circuit 200 of FIG. 2, the LCDpanel 430, row drivers 450, column drivers 452, a timing engine 456, anda video data engine 454. The video data engine 454 receives videoinformation via input video data 460. The video information includesrows of intensity data that form sequential video frames oftwo-dimensional intensity data. The video information also includestiming information delineating lines of intensity data within delineatedsequential video frames. The intensity data is transmitted to the videodata engine 454 and the timing information is transmitted to the timingengine 456. Timing information may also be transmitted to the video dataengine 454.

The timing engine 456 extracts line and frame timing information togenerate control signals for activating individual drivers within rowdrivers 450. For example, at the start of a new frame, and after a firstrow of intensity data is received by the video data engine 454, rowdriver 410 may be activated. After a second row of intensity data isreceived by the video data engine 454, row driver 411 may be activated,and so forth. Column drivers 452 receive analog input signals from thevideo data engine 454 to drive analog voltages on column wires withinthe LCD panel 430. When one row is activated, the column wires configurea row of LCD elements 440 to transmit an a corresponding set ofintensity values. Each one of the LCD elements 440 should be asubstantially identical instance of LCD element 440(0,0), described inFIG. 4A.

As an example of the operation of the LCD display device 290, when rowdriver 410 is active, column driver 420 drives LCD cell 432 within LCDelement 440(0,0) via column wire 424 with a voltage corresponding to adesired intensity for the LCD element 440(0,0). The voltage, inconjunction VCOM 290, determines an intensity value the LCD element440(0,0). In a color LCD panel 430, three LCD elements 440 (red, green,blue) are used to form one color pixel. In one embodiment, a software(SW) control signal 405 is used to drive signals SCL 252 and SDA 254, ofFIG. 2, to configure an output voltage value for VCOM 290. The SWcontrol signal 405 may be used during manufacturing to establish a valuefor VCOM 290. The value for VCOM 290 may be stored in non-volatilememory (not shown) within the LCD display device 290.

In sum, a technique for generating a digitally controlled voltage isdisclosed. The digitally controlled voltage is suitable for use as acommon voltage in LCD display panels. The technique is based on acircuit architecture comprising four resistors and a constant R DAC,such as a resistor string DAC. The four resistors are used to provideappropriate reference voltages to the constant R DAC. At least two ofthe four resistors are configured to be tunable in a layout modificationstep performed after conventional place and route physical design of arelated integrated circuit.

One advantage of the disclosed invention is that the circuitarchitecture is simplified with respect to conventional common voltagegenerators through the elimination of an operational amplifier.Furthermore, lower power and greater accuracy may also be achieved byeliminating the operational amplifier, which introduces an offsetvoltage. This offset voltage represents an error in the digitallycontrolled voltage output. Another advantage of the disclosed inventionis that the bias resistors may be configured to compensate forpost-layout routing resistance that deviates from nominal routingresistance values. Yet another advantage of the disclosed invention isthat dynamic power consumption within the constant R DAC may be reducedon average versus a conventional “R2R” resistor DAC because current flowthrough the constant R DAC remains essentially constant.

While the forgoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof. For example, aspects of thepresent invention may be implemented in hardware or software or in acombination of hardware and software. One embodiment of the inventionmay be implemented as a program product for use with a computer system.The program(s) of the program product define functions of theembodiments (including the methods described herein) and can becontained on a variety of computer-readable storage media. Illustrativecomputer-readable storage media include, but are not limited to: (i)non-writable storage media (e.g., read-only memory devices within acomputer such as CD-ROM disks readable by a CD-ROM drive, flash memory,ROM chips or any type of solid-state non-volatile semiconductor memory)on which information is permanently stored; and (ii) writable storagemedia (e.g., floppy disks within a diskette drive or hard-disk drive orany type of solid-state random-access semiconductor memory) on whichalterable information is stored. Such computer-readable storage media,when carrying computer-readable instructions that direct the functionsof the present invention, are embodiments of the present invention.

In view of the foregoing, the scope of the present invention isdetermined by the claims that follow.

The invention claimed is:
 1. An LCD display device comprising: aplurality of LCD elements to which is supplied a common voltagereference; and a common voltage reference generator including a constantresistance digital-to-analog converter having a resistor network forgenerating an output voltage from which the common voltage reference isgenerated, wherein the common voltage reference generator furtherincludes a resistor divider including a first resistor and a secondresistor connected in series between an analog reference voltage andground, a third resistor connected at a first end to a node between thefirst resistor and the second resistor and at a second end to theconstant resistance digital-to-analog convertor, and a fourth resistorconnected in series between the constant resistance digital-to-analogconverter and ground.
 2. The LCD display device according to claim 1,wherein the common voltage reference generator further includes anoutput driver for receiving the output voltage from the constantresistance digital-to-analog converter and generating the common voltagereference.
 3. The LCD display device according to claim 1, wherein theconstant resistance digital-to-analog converter further includes aplurality of gates that are controlled to generate the output voltage ata desired level.
 4. The LCD display device according to claim 3, whereinthe gates are controlled in accordance with a fixed-length integervalue.
 5. The LCD display device according to claim 3, wherein theconstant resistance digital-to-analog converter includes a resistornetwork.
 6. The voltage generator according to claim 3, wherein theconstant resistance digital-to-analog converter includes a linear stringof serially connected resistors.
 7. The voltage generator according toclaim 3, wherein the constant resistance digital-to-analog converterincludes a plurality of resistor segments, each having a string ofserially connected resistors.
 8. The LCD display device according toclaim 1, wherein the resistor network includes a linear string ofserially connected resistors.
 9. The LCD display device according toclaim 1, wherein the resistor network includes a plurality of resistorsegments, each having a string of serially connected resistors.
 10. TheLCD display device according to claim 1, wherein an electrical pathbetween an analog reference voltage and the constant resistancedigital-to-analog converter does not traverse an amplifier.
 11. The LCDdisplay device according to claim 1, comprising a first operationalamplifier having a first input coupled to an output of the constantresistance digital-to-analog converter and a first output coupled to afirst electrode of a first transistor, and a second operationalamplifier having a second input coupled to a second electrode of thefirst transistor.
 12. A method comprising: providing a plurality of LCDelements to which is supplied a common voltage reference; and providinga common voltage reference generator including a constant resistancedigital-to-analog converter having a resistor network for generating anoutput voltage from which the common voltage reference is generated,wherein the common voltage reference generator further includes aresistor divider including a first resistor and a second resistorconnected in series between an analog reference voltage and ground, athird resistor connected at a first end to a node between the firstresistor and the second resistor and at a second end to the constantresistance digital-to-analog convertor, and a fourth resistor connectedin series between the constant resistance digital-to-analog converterand ground.
 13. The method according to claim 12, wherein the commonvoltage reference generator further includes an output driver forreceiving the output voltage from the constant resistancedigital-to-analog converter and generating the common voltage reference.14. The method according to claim 12, wherein the constant resistancedigital-to-analog converter further includes a plurality of gates thatare controlled to generate the output voltage at a desired level. 15.The method according to claim 14, wherein the gates are controlled inaccordance with a fixed-length integer value.
 16. The method accordingto claim 14, wherein the constant resistance digital-to-analog converterincludes a resistor network.
 17. The method according to claim 14,wherein the constant resistance digital-to-analog converter includes alinear string of serially connected resistors.
 18. The method accordingto claim 14, wherein the constant resistance digital-to-analog converterincludes a plurality of resistor segments, each having a string ofserially connected resistors.
 19. The method according to claim 12,wherein the resistor network includes a linear string of seriallyconnected resistors.
 20. The method according to claim 12, wherein theresistor network includes a plurality of resistor segments, each havinga string of serially connected resistors.
 21. The method according toclaim 12, wherein an electrical path between an analog reference voltageand the constant resistance digital-to-analog converter does nottraverse an amplifier.
 22. The method according to claim 12, furthercomprising: providing a first operational amplifier having a first inputcoupled to an output of the constant resistance digital-to-analogconverter and a first output coupled to a first electrode of a firsttransistor; and providing a second operational amplifier having a secondinput coupled to a second electrode of the first transistor.
 23. Andevice comprising: a plurality of LCD elements to which is supplied acommon voltage reference; a common voltage reference generator includinga constant resistance digital-to-analog converter having a resistornetwork for generating an output voltage from which the common voltagereference is generated, wherein the common voltage reference generatorfurther includes a resistor divider including a first resistor and asecond resistor connected in series between an analog reference voltageand ground, a third resistor connected at a first end to a node betweenthe first resistor and the second resistor and at a second end to theconstant resistance digital-to-analog convertor, and a fourth resistorconnected in series between the constant resistance digital-to-analogconverter and ground; and an output driver for receiving the outputvoltage from the constant resistance digital-to-analog converter andgenerating the common voltage reference.
 24. The device according toclaim 23, wherein the output driver comprises a voltage-to-currentconverter circuit and a voltage driver circuit.
 25. The device accordingto claim 23, wherein the output driver comprises: a first operationalamplifier comprising a first input coupled the constant resistancedigital-to-analog converter and a first output; and a first transistorcomprising a first electrode coupled to the first output of the firstoperational amplifier.
 26. The device according to claim 25, wherein theoutput driver comprises: a second operational amplifier comprising asecond input coupled to a second electrode of the first transistor. 27.The device according to claim 23, wherein the resistor network includesa linear string of serially connected resistors.
 28. The deviceaccording to claim 23, wherein the resistor network includes a pluralityof resistor segments, each having a string of serially connectedresistors.
 29. The device according to claim 23, wherein an electricalpath between an analog reference voltage and the constant resistancedigital-to-analog converter does not traverse an amplifier.
 30. Thedevice according to claim 23, wherein an electrical path between ananalog reference voltage and the constant resistance digital-to-analogconverter does not traverse an amplifier.